1. Field of the Invention
The present invention relates to electrical insulating paper, and particularly to electrical insulating paper used while being immersed in electrical insulating oil and a stationary induction electrical apparatus using the electrical insulating paper.
2. Description of the Related Art
An oil-immersed transformer is a kind of stationary induction electrical apparatus. In the oil-immersed transformer, an iron core and a winding mounted on the iron core are immersed in electrical insulating oil in a tank, and electrical insulating paper (also simply referred to as insulating paper) is used as an insulating material such as insulating coating of a winding conductor. The insulating paper is a mat-like material having fine pores. It is known that the insulating paper exhibits an excellent insulating property while the pores are impregnated with electrical insulating oil (also simply referred to as insulating oil). The oil-immersed transformer is widely used as, for example, a transformer connected to an electrical system.
The insulating oil is classified according to a main component thereof. Examples of the main component used include mineral oil, alkylbenzene, polybutene, alkylnaphthalene, silicone oil, and ester oil. As the insulating paper, kraft paper, cellulose dielectric paper, chemical-added paper, synthetic fiber paper, or the like is used. Even at present, paper based on kraft paper is mainly used because of total convenience including costs and characteristics. Note that the insulating paper includes a press board here.
It is very important that the transformer connected to a power system have reliability and durability (long-term reliability). The oil-immersed transformer is deteriorated by time-related deterioration of the insulating oil and the insulating paper. Oxygen and water are mainly involved in the deterioration of the insulating oil and the insulating paper.
When the insulating oil is deteriorated, performance of the insulating oil can be recovered by a degassing filtering treatment or exchanging the oil for new oil. When the insulating paper is deteriorated, on the other hand, it is practically very difficult to exchange the insulating paper. Therefore, it is considered that the life of the oil-immersed transformer substantially depends on the deterioration of the insulating paper.
The insulating paper (other than synthetic fiber paper) widely used for the oil-immersed transformer or the like mainly includes cellulose. An average degree of polymerization of cellulose (average number of repetition of a glucose ring in cellulose) is used as an index of deterioration of the insulating paper. According to the Japan Electrical Manufacturers' Association Standard (JEM Standard, issued in 1993), use limit (life) of such insulating paper is about 450 in terms of the average degree of polymerization of cellulose.
Here, a decomposition reaction of cellulose as a main component of the insulating paper will be described briefly. Cellulose is a polymer material having glucose rings linearly polymerized, as illustrated in following chemical formula 1.

The decomposition reaction of cellulose ((C6H10O5)n) is roughly classified into three types, i.e., an oxidation reaction, a hydrolysis reaction, and a pyrolysis reaction. The oxidation reaction is a reaction in which a hydroxyl group (—OH) in cellulose is oxidized into a carbonyl group (—CO—) or a carboxyl group (—COOH) in the presence of oxygen to generate water (H2O), carbon dioxide (CO2), carbon monoxide (CO), and the like. The hydrolysis reaction is a reaction in which an ether bond (—O—) in cellulose is broken in the presence of water to generate carbon dioxide, carbon monoxide, a glucose molecule (C6H10O6), and the like. The pyrolysis reaction is a reaction in which a chemical bond in cellulose is broken by heat.
The pyrolysis reaction requires a higher temperature than the oxidation reaction and the hydrolysis reaction. Therefore, in a normal environment, the pyrolysis reaction is less likely to occur than the oxidation reaction and the hydrolysis reaction. The hydrolysis reaction breaks a main chain of cellulose. As a result, the hydrolysis reaction directly decreases the average degree of polymerization of cellulose. It is said that the carboxyl group generated in the oxidation reaction performs a catalytic action in the hydrolysis reaction. From these facts, it is said that the hydrolysis reaction of cellulose has a large influence on the time-related deterioration of the insulating paper. In other words, it is considered that durability of the insulating paper is enhanced by suppressing the hydrolysis reaction of cellulose (for example, enhancing water repellency of paper).
A variety of techniques for improving water resistance and heat resistance of cellulose are proposed. Examples thereof include a method for cyanoethylating or acetylating cellulose with acetonitrile. However, hydroxyl groups in cellulose form a hydrogen bond between molecules, and therefore have a low chemical reactivity. As a result, a complicated manufacturing process is necessary for the above-described method, and costs are thereby raised disadvantageously.
Meanwhile, there is a method for improving water resistance and heat resistance of cellulose without need of a chemical reaction with a hydroxyl group in cellulose. For example, JP 2003-082598 A discloses highly water-resistant paper provided with a surface coating film containing a polyol on at least one surface of a paper base material impregnated with an impregnating solution mainly containing polyisocyanate. According to JP 2003-082598 A, a bond having excellent chemical strength such as a urethane bond is generated between polyisocyanate with which the paper base material is impregnated and the surface coating film containing a polyol (between layers of the paper base material and the surface coating film). As a result, highly water-resistant paper having excellent water resistance and heat resistance is obtained.
JP 7-310300 A discloses base paper for a laminated plate. The base paper for a laminated plate includes a paper base material and a mixture containing a synthetic resin having a methylol group and a flame retardant containing methylolated phosphoric acid-dicyandiamide at a weight ratio of 1:0.4 to 2.0. The mixture is included in the base material at a content of 2 to 12% by weight with respect to an absolute dry weight of the base material, and is insolubilized. According to JP 7-310300 A, the synthetic resin and the flame retardant are subjected to a crosslinking reaction. As a result, the mixture containing the flame retardant is firmly fixed in the base paper. Flame-retardant base paper having excellent flame retardance and water resistance is thereby obtained.
JP 2012-219379 A discloses water-repellent printing paper. In the water-repellent printing paper, a support mainly containing pulp fiber is coated or impregnated with surface treating liquid containing at least a water-repellent agent and polymer latex having a hetero-phase structure. According to JP 2012-219379 A, highly water-repellent paper is obtained.